The first five minutes of Fish Tank contain more pure, spasmodic energy than most films do in their entirety. In the span of one sequence, our edgy heroine Mia (Katie Jarvis), wraps up a dance lesson, cusses her friend’s father, breaks her friend’s nose and then tries to free a horse from a squatter’s camp. Looking like a prepubescent Linda Fiorentino, Mia strides from one conflict to another, but what makes Fish Tank so relentlessly watchable is how Jarvis, while she might appear hellbent, is at the same time absolutely winging it. The sense of knowing it all and knowing nothing, of not being young but not being old enough — in other words, being a teenager — is captured to perfection in Fish Tank by director Andrea Arnold (Red Road) in what you might call the British Precious. Jarvis, like Gabourey Sidibe in Precious, makes her professional debut as Mia, making the project all the more remarkable.

Mia and her little sister Tyler (Rebecca Griffiths) live with their neglectful, party-girl mom Joanne (Kierston Wareing) in an Essex public housing complex. The low-income tenements, standing tall and forlorn, give the film its offbeat title, but Fish Tank is engineered to be claustrophobic. We’re always one step behind or beside Mia, the camera closely observing but never standing in judgment, even during Mia’s daily dance practice (she’s awful) or her frequent brushes with trouble. This is social realism in the vein of Mike Leigh’s Happy Go Lucky, another film with a perfect combination of clear, unfussy vantage points and an unbreakable lead actress. In both films, you can’t help wondering if there’s a man alive who can handle these women; in Fish Tank, he turns out to be Mia’s mother’s boyfriend. Slowly and carefully, with occasional moments of lyricism — note the way Mia’s chin rests on his shoulder, for example, or the way she sniffs the air around him — the handsome Connor (Michael Fassbender) gains Mia’s trust. This he accomplishes by not insulting her, first, then by asking her to dance for him.

As Mia’s relationship with her mother deteriorates, her friendship with Connor deepens. Fassbender, who appeared as the British film critic in Inglourious Basterds (think the biergarten scene), finally lands a leading role in Fish Tank, but despite his immersive ability he allows Jarvis to control the film. Theirs is an uneasy but totally credible pairing. With each gesture, you flinch at the implications of their friendship, at whether Connor is being merely kind or predatory. When it’s over, neither of them can go back to where they were, but only one of them will be moving forward.